1. Field of the Invention
The present invention relates to a semiconductor device such as a MOS field effect transistor having salicide layers or silicide layers and its manufacturing method.
2. Description of the Background Art
The semiconductor device represented by a SRAM and a DRAM has been recently advanced in integration, and multiple elements are mounted on one chip. Of these elements, transistors are mostly metal oxide silicon field effect transistors or MOSFETs, in particular. The MOSFETs are classified into the nMOSFET (negative MOSFET) in which electrons flow, and the pMOSFET (positive MOSFET) in which holes flow, and they differ in electric polarity, and a circuit is composed by a combination of the nMOSFET and the pMOSFET.
The field effect transistors are structurally classified into the surface channel type shown in FIG. 18 and the buried channel type shown in FIG. 19, and usually since the same gate electrode material is used in both the nMOSFET and the pMOSFET, the surface channel type is widely used in the nMOSFET and the buried channel type in the pMOSFET. Source/drain diffusion layers 1, 2 and a channel region of these transistors are formed by an ion implantation of impurities or a diffusion from a solid phase containing these impurities, and the n type diffusion layer contains phosphorus or arsenic as impurities, and the p type diffusion layer has boron or boron fluoride.
As the MOSFETs are becoming finer, a gate electrode 4 and the source/drain diffusion layers 1, 2 are also reduced in size, as a result of the reduction of their sectional area, the gate resistance and the diffusion resistance of the source/drain diffusion layers increase.
Against such an increase of those resistances, it has been attempted to lower the resistances by using a metal such as titanium, tungsten, cobalt and nickel, and forming compounds of the metal with a semiconductor (a silicon, etc.) in the gate electrode and the source/drain diffusion layers (to form silicides or salicides). At this time, in the case of a dual gate CMOS in which an n type gate and a p type gate differing in polarity are mutually connected, it is necessary to form silicides or salicides so as not to form a pn diode.
As the micronization is further promoted, lately, and the gate length and the like become much shorter, when a reducing metal is used such as titanium and tungsten, a silicide layer of high resistance in a metastable state is formed by the salicide forming mechanism depending on the gate length, and a silicide layer or a salicide layer of low resistance in a stable state cannot be formed.
It is hence required to form a silicide by using a non-reducing metal such as cobalt and nickel as diffusion seeds, which may realize silicide layers or salicide layers of low resistance.
When cobalt or nickel is used, however, the following problems are encountered.
A first problem is derived from the nature of diffusion seeds such as cobalt and nickel. That is, these metals are large in mobility contrary to titanium or tungsten, and they serve themselves as diffusion seeds, and form silicide layers. Accordingly, in the peripheral area of the silicide layer, the metal supply is decreased, and the film thickness of the silicide layer is smaller than that of the central area of the silicide layer. By contrast, in the central area of the silicide layer, cobalt or the like is supplied more than in the peripheral area, and the silicide forming reaction is promoted, the silicide layer becomes thicker, which may even exceed the thickness of the source/drain diffusion layers (for example, 0.1 .mu.m thick).
A second problem is derived from the non-reducing property of cobalt or nickel. Unlike titanium, these metals have no reducing action, and therefore if a natural oxide film is present on the surface of the gate electrode or the source/drain diffusion regions, the silicide forming is suppressed in the area, and the silicide forming reaction is promoted unevenly. As a result, as shown in FIG. 20, the flatness of the silicide layer 20 is poor, and the surface irregularity becomes large, and therefore the electric field becomes intense in the thick portion of the silicide layer 20, the interlayer is broken in the portion, and a leak current occurs. If the value of individual leak currents may be small, their sum may be too large to be ignored. In FIG. 20, meanwhile, the reference numeral 7P indicates a cobalt layer.
Thus, when forming a silicide by using a non-reducing metal such as cobalt and nickel, the presence of the natural oxide film or the like causes a junction leak. In the case of using a reducing metal such as titanium, on the other hand, oxygen is emitted outside by the reducing action in the silicide forming reaction, and the problem of the junction leak does not occur.